The invention concerns a method comprising measurement of at least one property of at least one fuel channel of fuel assemblies for nuclear boiling water reactors.
The core of a nuclear boiling water reactor (BWR) comprises a large number of fuel assemblies. The fuel assemblies extend in the vertical direction and are arranged parallel to each other in the core. FIG. 1 shows schematically an example of such a fuel assembly 8. The fuel assembly 8 comprises a number of fuel rods 10.
The bundle of fuel rods 10 which is in the fuel assembly 8 is at the sides surrounded by a casing 14. The casing is sometimes also called box or box wall. In the present application the casing is however called fuel channel 14. The fuel channel 14 surrounds the fuel bundle on all sides (except for upwards and downwards). Usually, the fuel assembly 8 is quadrilateral and the fuel channel 14 thus has a rectangular or square cross section shape. For the sake of clarity, in FIG. 1 the fuel channel 14 is partly removed on one side in order to show the fuel rods 10 that exist in the fuel assembly 8.
The fuel rods 8 consist of cladding tubes which contain nuclear fuel material, often in the form of pellets. In FIG. 1, a part of such a cladding tube is removed in order to show the nuclear fuel material 12. The fuel rods 10 are normally arranged parallel to each other in the fuel assembly 8. The actual nuclear fuel material 12 does not reach all the way up to the top of the fuel assembly 8 and also not always all the way down to the bottom of the fuel assembly 8. The length of the active part of the fuel assembly 8 is therefore shorter than the whole fuel assembly 8.
FIG. 2 shows schematically a cross-sectional view of four fuel assemblies 8. For the sake of clarity, in FIG. 2 only the quadrilateral fuel channels 14 of the fuel assembly 8 are shown. Each fuel channel 14 thus has four sidewalls. These have in one of the fuel assemblies 8 in FIG. 2 been numbered as 1, 2, 3 and 4.
As has been mentioned above, there is in the core of the nuclear reactor a large number of fuel assemblies 8 of the kind described above. Between certain of these fuel assemblies 8 there are, at least during a part of the time when the reactor is in operation, inserted control rods. Such a control rod 20 is shown very schematically in cross-section in FIG. 2. The control rod 20 has four control rod blades 22, 24, 26, 28 which contain a neutron absorbing material. The fuel channels 14 are usually made of a zirconium-based alloy. The control rod 20 is normally made of another alloy, often of stainless steel.
In the very particular environment which exists in the core of a nuclear reactor the components therein are influenced in different manners. Among other things, there is a tendency to the formation of hydrides in the fuel channels 14. Such hydride formation takes primarily place on the levels of the fuel channel 14 which correspond to the active area (where there is nuclear fuel) in the fuel assembly 8. Furthermore, oxides are formed on the fuel channels 14.
A particular phenomenon which is known within the field is so-called shadow corrosion. Shadow corrosion occurs in this particular environment, in particular on components of zirconium-based alloys when these components are arranged at a short distance from components of other materials, for example components of stainless steel. Shadow corrosion occurs in particular at a level where there is nuclear fuel in the fuel assembly, and where therefore a strong radiation is the case. Shadow corrosion can be seen as a dark area on the component which has been the subject of shadow corrosion. It is thus known that shadow corrosion may occur on the sidewalls 1, 4 of the fuel channels 14 which face a neighbouring control rod 20.
Shadow corrosion may cause different problems. Among other things, it has appeared that shadow corrosion may lead to the fact that the fuel channel 14 (and thereby the fuel assembly 8) will get bent. It is believed that this is due to the fact that shadow corrosion may lead to an increased hydrogen absorption, i.e. increased hydration, which in its turn may lead to hydrogen induced growth of the sidewalls 1, 4 of the fuel channel 14 which are the subject of shadow corrosion. This phenomenon is described in for example NRC Information Notice 89-69, Supplement 1: Shadow corrosion resulting in fuel channel bowing, United States Nuclear Regulatory Commission Office of Nuclear Reactor Regulation Washington, D.C. 20555-0001, 25 Aug. 2003.
The bow of the fuel assembly 8 may in its turn lead to further problems. For example, the bow may thereby be such that friction arises between the control rod 20 and neighbouring fuel assemblies 8. This may lead to the fact that it is not possible to withdraw the control rod 20 from the space between the fuel assemblies 8. Furthermore, the bow of the fuel assemblies 8 may lead to restrictions concerning allowed critical power (Critical Power Ration; CPR), which in its turn leads to the fact that the reactor cannot be operated with as high power as otherwise. This constitutes a high cost for the one which operates the nuclear power plant.
A further problem which has been noted is that even if the fuel assembly 8 in question only has been the subject of shadow corrosion from a control rod 20 during a part of its operation time, the shadow corrosion and the therewith associated increased hydrogen induced growth of the fuel channel 14 may lead to the fact that the fuel channel 14 becomes bent during later operation.
The phenomenon shadow corrosion and its influence on the fuel channels 14 during the whole life of the fuel assembly 8 is thus very complicated and not completely understood.
U.S. Pat. No. 5,889,401 and WO 00/34768 A1 describe methods and apparatuses for eddy current measurements on components, for example fuel rods, in nuclear power reactors. As has been mentioned in these documents, it may be important to be able to carry out measurements of for example the thickness of layers which may exist on such components. The layer may for example be an oxide layer. A measurement may also be done concerning other properties than layer thickness, e.g. concerning the content of hydrides in the component. It is convenient to carry out the measurement with a measurement probe which is arranged in the immediate vicinity of the measurement object.
WO 2007/053100 A1 describes a system which is suitable for carrying out eddy current measurements on components, such as fuel rods for nuclear reactors, which are located under water.